Electronic devices used with wireless communication systems, such as cellular phones, GPS receivers, and Wi-Fi enabled notebook and tablet computers, generally contain signal processing systems that have interfaces to the analog world. Such interfaces may include wire line and wireless receivers that receive transmitted power and convert the received power to an analog or digital signal that may be demodulated using analog or digital signal processing techniques. A typical wireless receiver architecture includes a low noise amplifier (LNA) that amplifies very small signals that may be received by an antenna and passes an amplified signal to later amplification and/or signal processing stages. By providing gain at the LNA, subsequent gain processing stages are made insensitive to noise, thereby enabling a lower system noise figure.
An LNA circuit generally contains at least one transistor and an input matching network. The purpose of the input matching network, which may be made of one or more passive devices such as inductors and capacitors, is to provide an impedance match and/or a noise match to a previous stage, such as an antenna, a filter, an RF switch, or other circuit. LNA implementations may also include an output matching network, a bias network, and other circuit structures.
In some wireless application scenarios where a wireless receiver is situated very close to a transmitter, for example, in the case of a Wi-Fi enabled device that operates in close proximity to a wireless router, the RF front-end of the wireless device may be exposed to very high signal levels. Thus, in order to maintain adequate linearity, the LNA may be configured to have variable gain or may be bypassed altogether in high signal environments. The additional complexity of providing a variable gain function to a LNA, however, poses additional challenges with respect to maintaining good noise and distortion performance over various gain settings.